Analog to digital converters (ADC's) are used in various electronic systems for converting analog signals to corresponding digital signals. An analog signal is any variable signal continuous in both time and amplitude, whereas a digital signal is represented by a series of data bits such as logical “0” and “1”. With analog signals, small fluctuations in the signal can imply meaning, but they may also be simply noise. With repeated transmission, duplication and processing noise elements can inadvertently become dominant. Translation of the analog signal to a digital signal can achieve both high fidelity and perfect reproduction capability and, as such, is often highly desirable.
One type of analog to digital converter (ADC) called a delta-sigma ADC (also known as a sigma-delta ADC) subtracts a feedback signal from the analog input signal to provide an error signal. The error signal is then quantified and filtered to form a digital output signal.
High-resolution analog to digital converters (ADC) are used in radar, missile and communication systems. For example, in receivers for radar systems, RF signals are downconverted to an intermediate frequency (IF), which are then fed into an analog to digital converter (ADC) for conversion into digital signals.
One problem with analog to digital converters (ADCs) is that large input signals can cause oscillation, or limit cycling, in the digital output signals, which can shut down the entire system. For example, in a radar receiver, large input signals (e.g., co-channel interference, large main bang leakage due to mistimed bases, or bright close range targets with improper gain control) can cause the analog to digital converter (ADC) to oscillate. The analog to digital converter (ADC) can remain in this state (i.e., oscillation of the digital output signals) indefinitely, or until another strong input signal initiates a normal operational state.
In order to alleviate oscillation, gain control circuits can be used to detect and process signals that are outside of an acceptable signal strength. For example, U.S. Pat. No. 7,088,794 to Nichols, and U.S. Pat. No. 6,191,725 to Lavoie disclose radar systems having gain control circuits. Another approach to this problem involves changing the architecture of the analog to digital converter (ADC) to suppress limit cycles.
Unfortunately neither of these approaches has achieved completely satisfactory results. A particular shortcoming is a significant degradation in receiver dynamic range.
Hence there is a need for a circuit that overcomes one or more of the technical problems as stated above.